1. Field of the Invention
This invention relates to a power supply circuit for a camera and more particularly to a power supply circuit which discontinues power supply to the electric circuit of the camera when the electromotive voltage of a power source has decreased.
2. Description of the Prior Art
There has been known a power supply circuit which controls power supply to a control circuit of a camera with a detection output obtained by detecting the voltage of a power source, in such a manner as to prevent erroneous action when the voltage of the power soure is below a value required for the operation of the control circuit of the camera, the power supply circuit serving to inhibit power supply to the control circuit when the power source voltage drops below a preset voltage level. However, in such a conventional circuit, the on-off operation of a power supply switching element connected to the power source and the control circuit is directly controlled by the output of a detection circuit which detects the power source voltage. Therefore, the power supply switching element is controlled by the output of the detection circuit which is produced the instant the power source voltage is impressed upon the detection circuit. Thus, battery check is performed and the power supply switching element is controlled according to the current which flows through the detection circuit the instant the power source voltage is impressed upon the detection circuit. On the other hand, the current which flows through the detection circuit the instant the power source voltage is impressed upon the detection circuit presents an unstable current value until the stabilization of the action of the detection circuit. In the conventional power supply circuit, therefore, accurate battery check can not be performed and this has been presenting a problem. Further, in the conventional power supply circuit, a battery check is performed with a detection level set at a fixed value irrespective of the kind of the battery employed as power source. Therefore, it is impossible to make a battery check that is suitable for the voltage drop characteristic of the battery. Referring to FIG. 1 of the accompanying drawings, when continuous discharge is made under the same lead, the discharge characteristic of some of small dry element batteries comes to show a sharp drop of their electromotive force at the end of discharge as represented by curves A and B while that of others shows a comparatively gradual voltage drop which starts from the beginning as represented by curves C and D. The former type of batteries includes mercury batteries, etc. The latter type includes alkaline manganese batteries, etc. With such a small battery used as power source for a camera, the discharge current allowed to flow to an electrical circuit is comparatively small. The discharge is an intermittent discharge. However, when a voltage that makes the electrical circuit, etc. inoperative, namely an inhibiting voltage, is set at 1.4 V for example, such inhibiting voltage can be appropriate for the batteries of the characteristic represented by the curves A and B. However, the batteries of the characteristic represented by the curves C and D will remain still usable even after the inhibiting voltage. Therefore, in cases where batteries of the characteristic curves C and D are to be used, a lower voltage should be selected as inhibiting voltage. On the other hand, with the inhibiting voltage set at a value (1.3 V for example) that is suitable for the characteristic curves C and D, if a battery of the characteristic curve A or B is used, the circuit comes into an inoperative region as the discharge characteristic of the battery rapidly drops after the value. Then, this causes imperfect control actions and results in unreliable photographing. It is therefore necessary that the inhibiting voltage which is used as reference for a battery check is altered according to the kind of the battery employed. For example, it is appropriate to set inhibiting voltage values V.sub.A, V.sub.B, V.sub.C and V.sub.D for batteries of characteristic curves A, B, C and D as illustrated in FIG. 1. However, as mentioned in the foregoing, in the conventional circuit, the battery check is performed with a fixed inhibiting voltage irrespective of variation in the kind of batteries and thus there has been the above stated problem. In addition to this, in the conventional power supply circuit, a detection circuit is constantly allowed to operate and the power supply is effected by the output of the detection circuit. Therefore, a great amount of power is consumed for battery check. Besides, when the power source voltage becomes lower than the inhibiting voltage during an exposure control action, the power supply then stops in the middle of the control operation. In such a case, the shutter might be kept open. Such has been another problem with the conventional power supply circuit.